In light of the increasing use of polymeric materials in vehicle manufacture, surface testing and analysis of such materials and components manufactured from such materials, is a current area of interest in the fields of material science and mechanics. The increased interest in evaluating scratch and/or mar resistance of polymers stems from the increasing use of polymers in various applications including, for example, windshields, appliances, vehicles and other durable goods.
In general, there are two basic types of material surface damage—mar and scratch. The term mar is generally used to refer to those marks caused by a contact with another object that are too shallow to be particularly noticeable under casual observation but nevertheless may become more noticeable when present in large quantities, under close inspection or when highlighted by other conditions. Examples of mar type damage include the damage commonly found on paint coats and dashboard surfaces that have suffered minor damage through contact with small objects such as stones, sticks, car doors or keys that have impacted the surface under evaluation. A scratch is a mark that forms visible grooves and/or surface damage, often referred to as “whitening” of the scratched surface when a lighter substrate is exposed by the removal of or damage to a darker surface layer or coating. A scratch is the more typical damage mode for those surfaces that are subjected to heavier sustained contact with an object that then moves relative to the surface under evaluation. This “whitening” of the scratched surface is a widespread damage mechanism that has prompted much concern in those industries and applications in which the surface condition and/or the residual strength of the damaged article may be a significant factor in a customer's long term satisfaction with the product and/or its residual value.
In order to evaluate the suitability of a composition and/or a particular component, a scratch and/or impact testing device may be applied to a surface or material under test using specified conditions, or a range of specified conditions, to evaluate the ability of the composition or component to resist scratches and/or mars. Analysis of the scratches and/or mars, both during and after the scratch test, may provide useful data and insight into the material properties or surface characteristics of the samples tested in order to guide further development.
Further, a better understanding of the micromechanical properties of materials, derived from surface testing and analysis, and a better understanding of the mechanical process of surface damage may enable quantitative evaluation in the scratch and/or mar behaviors of various materials under a variety of conditions. For example, scratch test data may, and hopefully will, reveal the load conditions under which the material under test begins to sustain and/or accumulate damage to its surface so that the composition and/or design may be adjusted accordingly to improve or maintain the ability of a given material or component to resist or withstand scratch and mar surface damage.
Although a number of conventional surface testing devices and methods have been developed and may provide some useful basis for comparison among various compositions and designs, many focus on relatively small “lab” samples. As a result, these devices and methods tend to utilize testing parameters that do not accurately simulate the damage modes that would be expected during the actual use of the materials and components. For example, some conventional testing methods and apparatus means may yield inconsistent and irreproducible data and results when they are applied to a component under test. For example, for parts that are heavily textured, the small stylus used in some conventional testing devices may be prone to “skip” or “bounce” during testing, thereby contributing to inconsistent testing results. In such instances, the inconsistent and irreproducible data and results complicate the ability of the engineers to produce a valid comparison between competing designs or compositions that would be helpful in guiding further development efforts.
Another concern relates to the range of loads and impact modes that can be applied effectively during scratch testing. In many instances, conventional bench testing is limited in its ability to simulate with a reasonable degree of confidence, the impact and contact conditions that would be reasonably expected to be endured by the materials/components under test during their normal service life. Further, many conventional devices may not allow for variable load or variable scratch speed testing during the completion of a single test. Still further, some conventional scratch testing devices may be unable to measure and capture quantitative data (e.g., load, scratch speed, scratch depth, etc.) during the actual surface test, producing instead a scratched and/or marred sample that is subjected to separate, and generally only qualitative, study. Without gathering quantitative data during testing, it may not be possible to verify that the intended load conditions and scratch speed actually occurred during testing, thereby rendering the test data less useful for comparison and decision making.
One such prior art test method is frequently referred to as the “Ford Five Finger” or “Five Arm” test that corresponds to standards developed for use in the automotive industry that generally corresponds to Ford's BN 108-13 standard, as well as General Motors' GMN3943 standard and Chrysler's LP-463DD-18-01 standard. One apparatus that may be utilized for conducting these tests is the Taber® 710 Multi-Finger Scratch/Mar Tester which includes a pneumatically driven, moveable sledge to which the test sample is mounted. The sledge moves in a linear fashion and allows for single or multiple pass testing and may be operated at various rates of speed relative to the marring tools. An assembly supports five independent splined-fingers that are typically configured to provide a constant, vertical load on interchangeable scratch pins (the scratch pins are typically to provide 1.0 mm or 7.0 mm diameter hemisphere contact surfaces). Individual weights of varying loads may be mounted to the top of each arm finger to exert a standard force on the surface of the test material. Each instrument may be used with a weight set that can include 2N-25N loads.
Although flat specimens up to 22 mm thick are normally tested, the “free-floating” arms enable testing of slightly contoured specimens provided they are rigid or adequately supported. A spring-loaded specimen holder is standard and can be mounted to the end or side of the moveable sledge for greater flexibility. To mount contoured or other non-standard specimens, an optional set of moveable hold-down clamps is available. As will be appreciated, pins having 1.0 mm or 7.0 mm diameter hemispherical contact surfaces, while perhaps delivering repeatable results, do not reflect the sort of damage modes that would be expected to affect polymeric components integrated into vehicles, particularly those incorporated to form exterior surfaces and, in particular, the load-bearing surfaces of pickup cargo beds.
Consequently, there is a need for improved apparatus and methods for surface testing and analysis. In addition, there is a need for surface testing devices and methods which produce reliable and consistent results. Further, there is a need for improved surface testing apparatus and methods that provide the ability to carry out multi-pass, load-controlled scratch tests with variable scratch speed and/or direction. Still further, there is a need for improved surface testing apparatus and methods that measure and capture more realistic damage modes during surface testing.